Polyisocyanates containing isocyanurate or biuret groups are known and disclosed, for example, in U.S. Pat. Nos. 3,487,080; 3,996,223; 4,324,879; and 4,412,073. While these polyisocyanates possess many outstanding properties which make them valuable components in high performance coatings and films, the viscosity of commercially available polyisocyanates containing isocyanurate or biuret groups must be reduced in order to decrease the amount of solvent which is necessary to obtain a suitable processing viscosity. The reduced viscosity requirement for the polyisocyanates is mandated by increasingly stringent government regulations regarding the limits on the amount of volatile solvents which may be present in coating systems. While polyisocyanates that contain primarily isocyanurate groups are typically less viscous than polyisocyanates which contain primarily biuret groups, their inherent viscosities are still higher than desired. Additionally, these isocyanurate group-containing polyisocyanates do not possess sufficient compatibility with highly branched polyesters and other polyfunctional coreactant resins to permit a significant reduction in the level of solvent(s) needed to achieve a useful coating formulation viscosity. However, both biuret and isocyanurate based polyisocyanates do possess an average isocyanate functionality generally greater than 3.0, more typically above 3.5. This results in a significant degree of crosslinking during the curing of coatings made from these polyisocyanates. Such crosslinking greatly enhances the mechanical performance and chemical resistance of coating films derived from these intermediates, relative to polyisocyanates of lower functionality that provide a lesser degree of crosslinking.
A method has been proposed in U.S. Pat. No. 4,801,663 to reduce the viscosity of isocyanurate group containing polyisocyanates prepared from 1,6-hexamethylene diisocyanate (herein also referred to as "HDI"). By terminating the reaction at a very low degree of trimerization, higher levels of the monoisocyanurate of HDI are obtained and the quantity of polyisocyanates containing more than one isocyanurate ring is reduced. The higher concentrations of monoisocyanurate obtained by the method of the '663 patent, coupled with corresponding lower levels of polyisocyanurates, is said to yield a lower viscosity product. Unfortunately, the method of the '663 patent provides relatively low conversion to the desired product and results in a substantial amount of unreacted HDI starting material. This must be separated from the product, thereby increasing the production cost, which more than offsets the small benefit of the slight viscosity decrease obtained. Additionally, it has now been found by the present inventors that the isocyanurate product prepared in accordance with the '663 patent does not provide a significant improvement in coreactant or solvent compatibility.
Recent disclosures in the patent literature have proposed the allophanatization of isocyanurates with monoalcohols as a route to low viscosity, economical polyisocyanates. By way of illustration, U.S. Pat. Nos. 5,124,427, 5,208,334, 5,124,427 and 5,258,482 disclose the formation of polyisocyanate mixtures which possess an NCO content of 10 to 47% by weight and viscosities of less than 10,000 cp. Typical NCO contents are about 20% with viscosities of about 520 to 1300 cp. These references do not teach the use of polyalcohols as reactants, apparently since these polyalcohols would not be expected to yield an allophanate modified polyisocyanurate polyisocyanate with a lower viscosity than an unmodified polyisocyanurate polyisocyanate at the same level of conversion.
As with pure isocyanurate group containing polyisocyanates, the allophanate modified isocyanurate based polyisocyanates also possess an average isocyanate functionality generally greater than 3.0, most typically above 3.5, which allows for a degree of crosslinking which significantly augments the mechanical performance and chemical resistance of coating films derived from them. While this is a significant advance, it is not an optimal one for aliphatic polyisocyanates. The products of these processes tend to have a lower isocyanate functionality, at the same level of conversion, than conventional isocyanurate polyisocyanates. Higher functionalities can be realized at higher conversions, but the viscosity of the product is increased. In addition, the lowest reported viscosity was 520 cp (U.S. Pat. No. 5,124,427). Such viscosities are still too high to achieve a dramatic reduction in a coating formulation's volatile organic compound content. In fact, no quantitative mention is made about the potential or actual reductions in volatile organic compound levels achievable by employing these allophanate modified isocyanurates. It is necessary to reach viscosities well below 1000 cp, and more preferably below 400 cp, to achieve significant reductions in a coating formulation's volatile organic compound content. It would be even better still if a viscosity below 1000 cp, and more preferably below 400 cp, could be achieved while maintaining an average isocyanate functionality generally greater than 3.0, more preferably greater than 4.0.
The preparation and use of polyisocyanates with viscosities below 500 cp containing uretidione and isocyanurate groups has been previously disclosed in DE-OS 1,670,667, DE-OS 1,670,720, DE-OS 1,954,093, U.S. Pat. No. 4,614,785 and U.S. Pat. Nos. 4,994,541, 4,614,785, 4,929,724 and 4,994,541. Specifically, for example, U.S. Pat. Nos. 4,614,785 and 4,994,541 describe the production of (cyclo)aliphatic polyisocyanates containing isocyanurate groups and uretidione groups by the use-of phosphine catalysts. The viscosities of the HDI derived products described were all below 200 cp. Still, improvements might be made. For example, the cycle times for the above reactions are longer than is desirable, making production costs relatively high compared to isocyanurate-based polyisocyanates. Unfortunately as well, the average isocyanate functionality of the resulting polyisocyanates containing uretidione and isocyanurate groups are generally 2.5 or less. This lower functionality results in less well crosslinked films for any given polyol resin compared to using a higher functionality polyisocyanate. Inferior mechanical properties as well as poorer chemical and environmental resistance result from employing these predominantly difunctional uretidione-containing polyisocyanates.
In other work, a high temperature, uncatalyzed process for producing substantially pure allophanates is described in U.S. Pat. No. 4,810,820. The '820 patent discloses that monols, diols, triols, or other such polyols can be heated to very high temperatures (&gt;150.degree. C.) with a (cyclo)aliphatic diisocyanate in the absence of a catalyst to form substantially pure allophanates. While the polyisocyanate products based on monoalcohols had low viscosities (generally less than 200 cp), their relatively low average isocyanate functionality would yield coatings with inferior properties. The described products based on diols and triols would have sufficient functionality to give high quality coatings, however those examples had very high viscosities (generally greater than 3,000 cp) and relatively low isocyanate contents (generally less than 18%). Unfortunately as well, the reaction conditions are very severe and difficult to control. A process that could be operated at a lower temperature (&lt;130.degree. C.) would be far more easily and safely controlled with less expensive equipment than is employed in the process of the '820 patent.
It would therefore be highly desirable if a polyisocyanate composition could be easily and safely produced by a low temperature process at a low cost to provide a product having a high average isocyanate functionality, as well as the desirable low viscosity properties of known polyisocyanates containing allophanate or uretidione groups. The present invention provides such a process and composition.